It is well known that metal coatings on substrates have wide applications, including protection from oxidative and corrosive attack, providing wear resistance and as electrical interconnects in electronic circuits and micro-electromechanical (MEMS) devices. There is also a wide diversity of known processes for coating metals on substrates, including electroplating, where many different metals may be deposited and for a large number of applications. In the electroplating process, the substrate typically is immersed in an aqueous bath containing metallic salts, and an electrical current drives the deposition of metals onto the substrate. Electroplating processes require that the substrate be part of the circuit. While the process is useful for the deposition of metals onto metals, it may also be depositing them onto upon non-conducting surfaces such as ceramics and plastics, but only after forming a seed metallic layer by alternative processes.
During electroplating the flow of electrons from the part to be plated through the solution to an anode determines where the metal ions deposit on the surface of the part. Because like charges repel one another, the electrons that try to flow from recess (the bottom of corners in a part) are retarded and electrons on the outside edges (points and the like) "see" fewer opposing electrons in close proximity, thus flow in greater numbers. The phenomenon produces thicker deposits at "high current density" points and thinner deposits at "low current density" sites. The work-piece to be coated is ordinarily limited to a size that fits within the dimensions of a bath.
Brush electroplating is a variant of electroplating which uses a tampon that is wet with electrolytic fluid and which is in electrical contact with the workpiece to locally deposit metal electrochemically. Electrochemical metallization is similar in that large surfaces are coated by moving a receptacle containing electrolytic fluid over a work-piece. In this case, the receptacle is part of an electrical circuit with the work-piece. Both brush electroplating and electrochemical metallization use substantially less quantities of fluid and generate less plating wastes than electroplating.
Electroless plating is a process in which metal ions in a dilute aqueous solution are deposited onto a substrate by means of a continuous chemical reaction. The process may also be defined as deposition by autocatalytic chemical reduction where the metal ions are reduced to metal by a chemical reagent. The reaction is dependent on catalysis by the surface of the workpiece, so that a coating forms upon it, while deposition elsewhere is minimized. For deposition to continue when the substrate is completely covered, the coating metal as well as the basis metal must be capable of catalyzing the surface, thus the term "autocatalytic". Electroless plating is most often done with nickel and copper, with some limited use with cobalt and precious metals. The plating is typically carried out in a bath so that the composition of the salts and temperature can be closely regulated. Electroless coatings are used in a wide diversity of industries including the chemical industry, electronics, automotive, aerospace and mining.
Electroless deposition takes place wherever the catalytic surface is wetted, which normally results in a more uniform deposit of metal compared to electrolytic plating. However, electroless processes are slower than electroplating. For thicker coatings, electroless deposition is often used as a pre-coating step followed by electrolytic deposition.
Electroless deposition processes are the subject of numerous patents. Not withstanding the number of such processes, they all appear to involve aqueous baths consisting of the metal salt, a reducing agent, a pH modifier, and complexing agents to optimize the deposition process. Several hundred different plating compositions have been reported for nickel alone and hundreds of others have been evaluated or used with copper or other metals. Because the electroless plating process "plates" by the chemical reaction of many components in the bath, control of the rate of plating is very difficult; therefore, time, temperature, the concentration of reducing agents, and the reaction products all affect the plating rate. For example, copper may be deposited at a rate of 0.03 to 0.06 microns/min at room temperature but an increase in temperature to 60.degree. C. is necessary to achieve rates of 0.15 microns/min (See Glenn O. Mallory and Juan B. Hajdu, Electroless Plating, Fundamentals and Applications, American Electroplaters Soc. 1990). However, if the rate is too fast, the bath may spontaneously decompose resulting in metal being plated all over the tank. If it is too slow, plating might cease or not initiate at all. For these reasons it is very difficult to predict or control electroless plating thickness. If the chemistry of the bath is not properly controlled, the physical characteristics of plated deposits may vary more than with electroplating baths. Complexing agents such as EDTA (ethylene diamine tetracetic acid) are often used to prevent premature deposition of metal particulates in the bath. For example, U.S. Pat. No. 5,248,527 provides a process for electroless plating tin, lead or tin-lead alloy on copper or copper alloy using an electroless plating bath containing a water soluble tin and/or lead salt, an acid capable of dissolving the salts, and a complexing agent. Also, numerous pre-processing techniques are available to prepare the surface of the work piece to accept the metal.
Electroless deposition creates decomposition products that contaminate the bath, requiring bath replacement and chemical waste disposal. Bath decomposition occurs even when plating is not taking place, and thus there is a need to recycle baths. For instance, in electroless nickel deposition, typical reducing agents are hypophosphites or borohydrides, which form phosphides or borides to drive the reaction. In hypophosphite reduction of nickel salts, the build-up of orthophosphite ions results in slow deposition and poor quality of deposit. Therefore, the life of any electroless nickel bath is limited to a few turnovers, for example, approx. 10-12. Consequently, these baths are treated as a waste product at regular intervals and there is considerable environmentally objectionable waste that is produced. Many of these baths also create hazardous vapors, particularly when the baths are heated to increase plating rate. Further, the electroless nickel coatings, which have phosphide or boride inclusions are hard and useful for wear applications, but they have low ductility and are brittle. To form ductile nickel coatings, electrolytic rather than electroless plating is used.
Electroless processes have also been used in depositing metal onto plastic. In this case, the surface of the plastic is modified to achieve adhesion by (a) mechanical methods resulting in a rougher surface providing better anchoring sites for the coating, (b) surface modification by chemical methods improving bond formation, (c) by bombardment with high energy particulates, or combinations of the above methods. Etching involves chemical removal of one of the components of a resin while maintaining the physical integrity of the bulk resin. When this is achieved, a mechanical bond results. Chemical bonds, or bonds without etch porosity, contribute to adhesion but by themselves are insufficient and often fail in wet environments particularly when the part is thermally stressed. After etching, a catalyst, tin-palladium is absorbed into the etched surface so as to initiate electroless metal deposition.
Many processes are known for depositing precious metal catalysts onto non-conductor substrates to facilitate adhesion of the subsequent metallic deposit. For example, U.S. Pat. Nos. 4,136,216, 4,151,311 and 4,233,344 which describe processes that use colloids of non precious metals, formed in solution and precipitated onto non-conducting surfaces in lieu of precious metals. The colloidal copper that is present only to a very small extent serves as an activator and not the source of the metal deposit. U.S. Pat. No. 5,492,613 discloses a process for electroless plating a metal on the surfaces of nonconductive material substrates by brushing or spraying composite chemical solutions to activate the surface. The surface is subjected to mechanical or chemical agent solution pretreatment to activate the surface to form catalyst sites. Copper is then deposited by brush electroplating or by immersing in electrolytic or electroless baths.
U.S. Pat. No. 5,759,230 discloses a method of forming nanostructured metallic films of tungsten, titanium, molybenum, rhenium and tantalum by reaction of a metal salt which is soluble in an alcohol, and heating the substrate, generally to about 120-200.degree. C. with the solution to deposit films with nanosize grains. U.S. Pat. No. 4,539,041 describes a process for reducing in a liquid phase a solid compound selected from the oxide, hydroxide or salt of a metal selected from the group consisting of gold, palladium, iridium, osmium, copper, silver, nickel, cobalt, lead and cadmium, which comprises heating said solid compound suspended in a polyol and thereafter isolating the formed metallic precipitate.
U.S. Pat. No. 5,529,804 describes a process for the production of hard materials wherein hard constituent powders are coated with cobalt and/or nickel metal in solution by heating the metal containing powders in a polyol and boiling for many hours, reducing the metals. The source of the metal is an oxide, hydroxide or salt. The polyol functions both as a solvent and a reducing agent at the same time and is present in an amount of at least 5 times more moles of polyol than moles metal. It is stated that an even distribution of the cobalt and/or nickel is achieved over the surface of the hand constituent powder without the formation of islands of pure metal.
U.S. Pat. No. 4,098,922 describes a method of depositing a metal on a substrate which comprises coating the substrate with a sensitizing solution comprising a reducible salt of non-noble metal, a primary reducing agent selected from the group consisting of 2,7 anthraquinone disulfonic acid, alkali metal salts of 2,7 anthraquinone disulfonic acid and mixtures thereof, and a secondary reducing agent which can be a polyol. The substrate is then dried and stored for at least 3 days and exposed to ultraviolet light to form a catalytic layer.
Methods of producing structures of nanophase materials are being sought because they have an array of novel attributes. For example, nanophase copper is five times stronger than the ordinary metal [2]. Unfortunately it is difficult to prepare fully dense (non-porous) nanostructures by powder metallurgical means because consolidation by heat and/or force causes recrystallization and grain growth. Methods of forming nanocrystalline grain structures at room temperature are desired to circumvent the grain growth that occurs during densification.
The object of the present invention is to provide a new electroless method of deposition that overcomes the limitation of the numerous prior art methods. It is a further object of the invention to provide electroless deposition of metals onto substrates which heretofore have not been amenable to electroless process. A yet further object of the invention is to provide a fast, economical method for placing a metallic coating on a substrate.